Our Star

© 2004 Pearson Education Inc., publishing as Addison-Wesley

Why Does the Sun Shine? Our goals for learning:

• What process creates energy in the Sun?

• Why does the Sun’s size remain stable?

• How did the Sun become hot enough for fusion in the first place?

© 2004 Pearson Education Inc., publishing as Addison-Wesley

The Sun’s Energy Source

• The first scientific theories involved chemical reactions or gravitational collapse.

• chemical burning ruled out…it can not account for the Sun’s luminosity • conversion of gravitational potential energy into heat as the Sun contracts would only keep the Sun shining for 25 million years • late 19 th -century geological research indicated the Earth was older than that • Development of nuclear physics led to the correct answer • the Sun generates energy via nuclear

fusion

reactions • Hydrogen is converted into Helium in the Sun’s core • the mass lost in this conversion is transformed into energy • the amount of energy is given by Einstein’s equation: E = mc 2 • given the Sun’s mass, this will provide enough energy for the Sun to shine for 10 billion years © 2004 Pearson Education Inc., publishing as Addison-Wesley

Striking a Balance

• The Sun began as a cloud of gas undergoing gravitational collapse.

• the same heating process, once proposed to power the Sun, did cause the core of the Sun to get hot & dense enough to start nuclear fusion reactions • Once begun, the fusion reactions generated energy which provided an outward pressure.

• This pressure perfectly balances the inward force of gravity.

• deep inside the Sun, the pressure is strongest where gravity is strongest • near the surface, the pressure is weakest where gravity is weakest • This balance is called

gravitational equilibrium

.

• it causes the Sun’s size to remain stable © 2004 Pearson Education Inc., publishing as Addison-Wesley

Composition of the Sun

0.8% 28% 70%

H He O C Fe

0.3% 0.2% © 2004 Pearson Education Inc., publishing as Addison-Wesley

Composition of the Sun

We know this by identifying the

absorption lines

in the Sun’s spectrum.

These lines are formed in the photosphere.

© 2004 Pearson Education Inc., publishing as Addison-Wesley

Layers of the Sun

© 2004 Pearson Education Inc., publishing as Addison-Wesley

Core

• T = 1.5 x 10 7 K; depth = 0 – 0.25 R  • This is where the Sun’s energy is generated.

Interior Zones

• T < 8 x 10 6 K; depth = 0.25 – 0.86 R  • Energy is transported through the interior.

• The interior is divided into two zones: • Radiation Zone • Convection Zone • Boundary between them is at: • T = 2 x 10 6 K; depth = 0.70 R  © 2004 Pearson Education Inc., publishing as Addison-Wesley

Photosphere

• T = 5,800 K; depth = 400 km • This is the yellow “surface” that we see.

© 2004 Pearson Education Inc., publishing as Addison-Wesley

Chromosphere

• T = 1 – 5 x 10 4 K; depth = 2,500 km • A thin layer above the photosphere where most of the Sun’s UV light is emitted.

• UV image of the Sun • light emitted from neutral Helium at 20,000 K © 2004 Pearson Education Inc., publishing as Addison-Wesley courtesy of SOHO/SUMER consortium SOHO is a project of ESA and NASA

• T = 2 x 10 6 K; depth 

Corona

600,000 km • The hot, ionized gas which surrounds the Sun.

– it emits mostly X-rays • It can be seen in visible light during an eclipse.

X-ray image (YOHKOH telescope) © 2004 Pearson Education Inc., publishing as Addison-Wesley Visible image

The Cosmic Crucible Our goals for learning:

• Why does fusion occur in the Sun’s core?

• Why is energy produced in the Sun at such a steady rate?

• Why was the Sun dimmer in the distant past?

© 2004 Pearson Education Inc., publishing as Addison-Wesley

Why does fusion occur in the Sun’s core ?

• Nuclear fusion • a reaction where heavier nuclei are created by combining (

fusing

) lighter nuclei.

• all nuclei are positively charged • Electromagnetic force causes nuclei to repel each other.

• for fusion to occur, nuclei must be moving fast enough to overcome E-M repulsion • this requires high temperatures & pressures • When nuclei touch, the nuclear force binds them together © 2004 Pearson Education Inc., publishing as Addison-Wesley

What is nuclear fission ?

• Nuclear fission • a reaction where lighter nuclei are created by splitting heavier nuclei.

Only fission has been achieved by mankind It is easier to destroy than to create.

© 2004 Pearson Education Inc., publishing as Addison-Wesley

Sun fueled by a process called the Proton-Proton Chain IN: 6 H, (2 e ) OUT: He, 2 H, 2  e , 4  Effectively 4 H nuclei are converted into 1 He nucleus and energy is released. © 2004 Pearson Education Inc., publishing as Addison-Wesley

Why does the Sun Shine ?

But….

mass of He = 99.3% of 4 x mass of H where did the other bit of mass go?

into energy!!!

E = mc

2 © 2004 Pearson Education Inc., publishing as Addison-Wesley

The Solar Thermostat

• The rate of fusion reactions depends on temperature.

• the higher the T, the faster the rate, the more energy is produced • This fact, coupled with gravitational equilibrium, acts as a mechanism which regulates the Sun’s energy output.

• its energy output (luminosity) remains stable © 2004 Pearson Education Inc., publishing as Addison-Wesley

The Solar Luminosity

• The Sun’s luminosity is stable over the short-term.

• However, as more Hydrogen fuses into Helium: • four H nuclei convert into one He nucleus • the number of particles in Sun’s core

decreases

with time • the Sun’s core will contract, causing it to heat up • the fusion rate will increase to balance higher gravity • a new

equilibrium

is reached for stability at a higher energy output • the Sun’s luminosity increases with time over the long-term • Models indicate the Sun’s luminosity has increased 30% since it formed 4.6 billion years ago.

• it has gone from 2.9 x 10 26 watts to today’s 3.8 x 10 26 watts © 2004 Pearson Education Inc., publishing as Addison-Wesley